A diminished visual acuity or total loss of vision may result from a number of eye diseases or disorders caused by dysfunction of tissues or structures in the anterior region of the eye and/or posterior region of the eye. The eye is divided anatomically into an anterior and posterior segment. The anterior segment includes the cornea, anterior chamber, iris and ciliary body (anterior choroid), posterior chamber and crystalline lens. The posterior segment includes the retina with optic nerve, choroid (posterior choroid) and vitreous. The posterior portion of the eyeball supports the retina, choroid and associated tissues.
Protein conformational disorders are a set of inherited human diseases in which mutant proteins are misfolded. Protein conformation disorders can affect proteins of the vertebrate visual system and cause dysfunction of tissues or structures within the eye. For example, misfolded proteins can aggregate, causing damage within cells expressing the mutant protein. Certain types of retinitis pigmentosa are associated with protein conformational disorders. For example, many opsin mutants associated with retinitis pigmentosa are misfolded and retained within the cell.
Rhodopsin (Rh) is the visual pigment protein of the rod cell and belongs to a large family of transmembrane G protein-coupled receptors, which are involved in numerous physiological functions. This superfamily of membrane glycoprotein receptors is characterized by seven transmembrane α-helices that are anchored within the lipid bilayer. Rhodopsin is composed of opsin, the apoprotein, which is bound to chromophore. In contrast to other G protein-coupled receptors, which respond to diffusible ligands, rhodopsin responds to light. The chromophore in rhodopsin, 11-cis-retinal, is a covalently bound reverse agonist that yields a distinct UV-visible spectrum with a max of approximately 500 nm. Rhodopsin is approximately 40,000 daltons. The crystal structure of rhodopsin has been previously elucidated.
In recent years, more than 100 opsin mutants have been linked to various genetic forms of retinitis pigmentosa, the most common form of hereditary retinal degeneration. Retinitis pigmentosa leads to photoreceptor death and subsequent severe loss of peripheral and night vision. These opsin mutants account for nearly 50% of all the autosomal dominant retinitis pigmentosa cases with the most common being a substitution of Proline 23 to Histidine (P23H), which accounts for approximately 10% of all retinitis pigmentosa cases.
Although opsin mutants display distinct biochemical features, the mutant phenotypes fall mainly into three basic classes. Class I mutants are expressed at nearly wild-type levels and form stable pigment with 11-cis-retinal in the dark. The associated amino acid substitutions cluster at the C terminus of rhodopsin and disrupt vectorial transport to the rod outer segment. Some mutants also inefficiently activate transducin. Class II mutants do not bind 11-cis-retinal and are retained in the endoplasmic reticulum. Class III mutants, like P23H, form small amounts of pigment and mainly remain in the endoplasmic reticulum or form aggresomes. These mutants are typically targeted for degradation by the ubiquitin proteasome system. Other phenotypic properties associated with opsin amino acid substitutions may include the destabilization of the structure formed within the rod outer segment by mutated rhodopsin molecules or the disruption of other biological processes unique to the rod outer segment.
Patients with the P23H substitution usually have milder disease progression than those harboring other rhodopsin mutations. Based on the crystal structure of rhodopsin, Proline 23 is located in the N-terminal tail within one of the β-strands that make up an integral part of the N-terminal plug. The plug keeps the chromophore in its proper position, and mutations within this region result in improper folding of opsin and poor binding of the chromophore. P23H-opsin forms the pigment poorly, does not acquire the Golgi-related glycosylation, and is retained within the cell, collectively providing evidence that it is misfolded.
There is currently no effective treatment for protein conformational disorders of vertebrate visual systems. The present invention satisfies this and other needs.